As for an image-forming element utilizing a cobalt complex, there are known elements including those described in, for example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP OPI Publication) Nos. 50-139722/1975, 50-139723/1975 and 50-139724/1975. Those elements described therein are excellent in such a point that an image can be formed in a relatively simple dry process. However, the light-sensitivity of such a cobalt complex as mentioned above is so limited in itself that it can only be light-sensitive to rays of light having a wavelength shorter than 300 nm. In an image-forming element such as those described in the above-given publications, therefore, a photoinduced reducing agent such as a naphthoquinone derivative and a sensitizer is used as a light-sensitive substance, so that the light-sensitive elements are provided, by the functions of the photoinduced reducing agent, with a sensitivity to rays of light having the wavelengths than 300 nm. However, it cannot be said that the sensitivity thereof is satisfactorily high and, further, those light-sensitive elements have the defect that the preservability of the elements deteriorates when spectrally sensitizing them to a long wavelength of light, and that a stain is produced by a photoinduced reducing agent after forming an image. In addition to the above, those light-sensitive elements are limited to their own application, because, when making an exposure, a scanning-exposure by a semiconductor laser beam cannot be applied which has remarkably been developed in recent years. JP OPI Publication No. 59-95529/1984 discloses a method in which a silver salt capable being dry-processed is used with a cobalt complex for making a sensitivity higher. However, the above-mentioned dry-process has the defect that a stain produced by a developer for a silver salt, because, in this method, an image is produced so that a dye is so released as to meet the development of the silver salt.
As for the other image-forming elements each utilizing a cobalt complex, JP OPI Publication No. 57-139739/1982 discloses a light-sensitive element utilizing the borate complex of cobalt, and JP OPI Publication No. 2-182701/1990 discloses another light-sensitive element containing a complex comprising a cation of a transition metal including cobalt and a borate anion. The former complicates the process thereof, because development is carried out with an ordinary silver halide photographic color-developer. The latter is subjected to a copolymerization process for forming an image, and is not preferable to obtain an image having a contrast gradation.